Technical Field
The present disclosure relates to a resonant transducer, and more particularly to an electrode separating structure which ensures insulation between electrodes.
Related Art
A resonant transducer is a device which detects a change of a resonance frequency of a resonator formed on a silicon substrate to measure an applied physical quantity. The resonant transducer is widely used for a transmitter or the like as MEMS (Micro Electro Mechanical Systems) devices such as a pressure sensor, an acceleration sensor, an angular velocity sensor, an oscillator, etc.
In the resonant transducer, a plane side of a long plate shaped resonator both ends of which are fixed is formed in parallel with a silicon substrate and the resonator is vibrated in a vertical direction of the silicon substrate. However, patent literature 1 discloses that a plane side of a resonator is formed vertically to a silicon substrate and the resonator is vibrated in a transverse direction of the silicon substrate. Thus, a manufacturing process is simplified, so that the resonant transducer can be highly accurately and inexpensively manufactured.
FIG. 18 is a diagram for explaining a structure of a resonant transducer in which a plane side of a long plate shaped resonator is formed vertically to a silicon substrate to vibrate the resonator in a transverse direction of the silicon substrate. FIG. 18 shows a sectional view of a resonator part.
As shown in FIG. 18, in the resonant transducer 300, an SOI substrate has a structure in which a BOX layer 311 made of an oxide film is inserted between the silicon substrate 310 and a surface silicon layer (a lower part of an active layer 320). On the SOI substrate, the resonator 330, a first fixed electrode 341 and a second fixed electrode 342 are formed by processing the active layer 320 made of silicon. An oxide film 360 and a shell 351 made of polysilicon 350 are formed.
The first fixed electrode 341 and the second fixed electrode 342 are formed so as to sandwich the resonator 330 between them. In a periphery of the resonator 330, a vacuum chamber 370 is formed. Electrodes are also formed in end sides of the resonator 330, which are not shown in FIG. 18, and function as resonator electrodes 331.
FIG. 19 is a diagram showing an example of a mask pattern of the active layer 320. The first fixed electrode 341 and the second fixed electrode 342 are electrodes which apply positive and negative bias voltages having the same magnitude or level. The resonator electrodes 331 are electrodes which apply an ac signal of a frequency equal to a resonance frequency of the resonator 330. In this case, however, combinations of voltages and signals applied respectively to the electrodes may be changed. Further, the number of the fixed electrodes may be sometimes set to one.
Since the resonator 330 and the resonator electrodes 331 need to be insulated from the first fixed electrode 341 and the second fixed electrode 342, in the mask pattern, electrode separating gaps WH are provided between the first fixed electrode 341 and the resonator electrodes 331. The electrode separating gaps WH are also provided between the second fixed electrode 342 and the resonator electrode 311. Between the first fixed electrode 341 and the resonator 330, resonator gaps WV are provided. The resonator gaps WV are also provided between the second fixed electrode 342 and the resonator 330. Since an etching rate of a dry etching for separating the active layer 320 is set to be the same rate, the electrode separating gap WH and the resonator gap WV have the same width.
FIGS. 20A to 20C and FIGS. 21A to 21C are diagrams which explain manufacturing processes of the resonant transducer 300 and show an A-A section including the electrode separating gap WH and a B-B section including the resonator gap WV in FIG. 19.
As shown in FIG. 20A, on the initial active layer of the SOI substrate including the substrate 310, the BOX layer 311 and the active layer, an epitaxial growth of a silicon layer including high concentration boron is made to form the active layer 320.
Then, the dry etching of the active layer 320 is carried out by using the above-described mask pattern. As a result, as shown in FIG. 20B, trenches which separate the active layer 320 are formed.
In order to carry out a photolithography with a line narrow in width which forms an etching channel in a manufacturing process afterward, a surface of a wafer needs to be kept flat. Thus, as shown in FIG. 20C, the trenches which separate the active layer 320 are buried by the oxide film 360. The trenches are buried by the oxide film 360 through plasma CVD or an LP-CVD. However, since opening parts of the trenches are narrow, voids (spaces) are ordinarily formed in the buried parts.
Further, as shown in FIG. 21A, a film of polysilicon 350 is formed and the surface of the wafer is flattened. Then, as shown in FIG. 21B, the etching channel is formed in the vicinity of the resonator 330. The etching channel serves as an introducing opening of etching liquid or etching gas for removing the oxide film 360 in the periphery of the resonator 330, that is, a sacrifice layer.
Then, as shown in FIG. 21C, the sacrifice layer is etched by using the etching channel to release the resonator 330. After that, a vacuum seal is made by forming the polysilicon film under a prescribed environment to bury the etching channel and form the vacuum chamber 370. Further, holes and electrode pads are formed through which the electrodes are respectively allowed to come into contact. Thus, the resonant transducer 300 shown in FIG. 18 is manufactured.